1. Field of the Invention
The invention relates to the field of semiconductor random access memories.
2. Prior Art
Semiconductor memories including random access memories (RAMs), read-only-memories (ROMs), programmable read-only-memories (PROMs) are known in the prior art. These memories have been fabricated in the prior art utilizing MOS technology wherein an entire memory is fabricated on a single silicon substrate. It is a characteristic of such fabrication that production yields are substantially unaffected by the density of devices. Thus, for a fixed area of substrate it is desirable to provide as many memory cells as possible in order to obtain the lowest costs per bit of storage.
In semiconductor MOS RAM's cells are either static or dynamic. The static type of cells generally include bistable circuits such as flip-flops which once set in a particular set, remain in that state. Such circuits require a number of devices, for example, several field effect transistors are required in a flip-flop. The dynamic memory cells typically employ capacitive storage but since such storage is transient, refreshing is required. The capacitive storage means used in the prior art include the gates of field effect transistors, junction capacitance, etc. Examples of such dynamic prior art cells are shown in U.S. Letters Pat. 3,593,037 and 3,706,079.
In order to obtain the high densities, the present invention utilizes memory cells employing only a single active device. The device is a field effect transistor which is used to gate or select a capacitive storage means. Such single device cells utilizing field effect transistors have been known in the prior art. For example, see U.S. Letters Pat. No. 3,387,286. Another so-called "one device per bit" capacitive storage memory array utilizing field effect transistors is shown in U.S. Letters Pat. No. 3,699,537. Other prior art is shown in U.S. Letters Pat. No. 3,533,089 and 3,514,765. The present invention utilizes the single active device per cell concept in a unique manner to provide a practical memory system.